Mining of taconite ores using high frequency magnetic energy



Nov. 11, 1958 H. L TOUR x-:rAL 2,859,952

MINING F TACNITE ORES USING HIGH FREQUENCY MAGNETIC ENERGY Filed sept. s, 1951 2 .sheets-sheet 1 had @ZUM ATTORNEYS.

Nov. 1.1, 1958 A TOUR ETAL 2,859,952

H. L MINING OF' TACONITE ORES USING HIGH FREQUENCY MAGNETIC ENERGY Filed Sept. 8, 1951 2 Sheets-Sheet 2 A'r TORNEYS.

United States Patent C MlNlNG F TACONITE GRES USING HIGH FREQUENCY MAGNETIC ENERGY Application September 8, 1951, Serial No. 245,682

12 Claims. (Cl. 262-1) This invention relates to the mining, fracturing and beneciating of ores, and particularly of hard silicious rock containing a low percentage of magnetic iron ox* ide. Since the richer iron ores have been fairly well exhausted it has become increasingly necessary to mine ore less rich in iron. Various procedures have been devised for beneciating such ores so as to obtain material suitable for charging into a blast furnace.

Among ores which had been mined in recent years are the so-called Taconite ores which are very hard silicious rock containing particles of magnetic iron oxide sometimes distributed throughout the matrix and sometimes concentrated in irregular roughly parallel bands in the mass of the ore. This ore is a relatively low grade ore and generally contains only from 2O to 35 percent iron. lt must be concentrated in order to make it suitable for blast furnace use. It must be crushed to the point where approximately 85 percent will pass through a 325 mesh screen.

ln accordance with present practice, the mining operation involves stripping off the over-burden and then piercing holes so that blasting charges may be inserted. Because of the hardness of the rock it has been necessary to use a jet-type drill, using oxygen, kerosene and water. A unit of this type is extremely expensive and can only drill about a hundred feet of six inch diameter hole per day. This procedure is disadvantageous and is inconvenient and expensive to use in cold weather because apparatus must be provided to prevent the cooling water from freezing and interfering with the operation of the jet drill.

Blasting charges are inserted in the holes and exploded to produce large boulders. The boulders broken off as a result of the blasting operation may weigh several tons each. The disadvantages of the procedure are further observable when it comes to the problem of transporting these boulders to a primary crushing plant. Such a crushing plant must be of tremendous size and again requires an extremely expensive installation.

With the foregoing diiliculties and objections in mind it is an object of our present invention to provide a novel method of mining which is operable regardless of weather conditions. lt is another object of our invention to provide a mining procedure whereby the ore as mined will not be in large unwieldy boulders but in relatively small fragments.

it is another object of our invention to provide for a method of successively fracturing and comminuting the ore as a preparation for beneliciating processes. Yet another object of our invention is to provide a novel method of beneiciation by selective fracturing. Further objects of our invention involve the provision of mining machinery for use with the method of the present invention and the provision of beneficiating and fracturing apparatus for use with our novel method.

The foregoing objects Vand others which will be pointed ont in more detail hereinafter or which will be apparent 2,859,952 Patented Nov. 1l, 1958 ICC to those skilled in the art upon reading these specifications, we accomplish by that certain construction and arrangement of parts and by that series of method steps of which we shall now describe certain exemplary embodiments.

Reference is made to the drawings forming a part hereof and in which:

Figure l is a perspective view of a fragment of Taconite ore showing an induction loop by means of which the rock is fractured.

Figure 2 is a perspective view of the same fragment after it has been subjected to the high frequency energy.

Figure 3 is an elevational view of a fracturing or beneciating apparatus according to our invention.

Figure 4 is a fragmentary perspective view of a slightly modified form of apparatus.

Figure 5 is a diagram useful in connection with an understanding of the principles of our invention.

Figure 6 is a perspective View of a mining operation showing the machinery in operation.

Typical rocks have a compressive strength of approximately 9500 pounds per square inch, as compared with a tensile strength of approximately 350 pounds per square inch. All present day crushers of which we are aware depend upon compression to fracture the rock. The great divergence between the tensile strength and compressive strength suggests that the force required in tension fracture might be as little as '5.7 percent of that required for compression fracture. it indicates also that the energy requirement for tension fracture might be as low as 0.14 percent of the energy required for compression fracture.

In order to produce a system of forces within a piece of rock which will result in fracture, an expansion or contraction must be caused locally in such a manner as to be restrained by adjacent material in which tensile or compressive forces are induced. The tensile forces induced must be large enough to exceed the tensile strength of the material. ln other words, the strains resulting from the tensile stresses must exceed the fracture strain of the material.

Referring to the diagram of Figure 5, there must be some restraint in order to produce a fracture strain. lf the legs A and C in Figure 5 are caused to expand by internal volumetric changes, the leg B would resist the expansion of the legs A and C and would be subjected to tensile stresses while the legs A and C would be subjected to compressive stresses. Under such a set-up the leg B would be the restraining member. Thus if the legs A and C were heated so as to cause them to expand and the leg E remained cold, the leg B would be the restraining niemeer. lt will be clear that if all legs were heated at the same time to the same temperature no restraint would be present.

ln addition to the strains in legs A, B, and C, bending is induced in legs D and E which produces additional tensile and compressive strain. Since the materials in question are much weaker in tension than in compression it is the tensile strength that governs fracture.

By analogy to the diagram referred to above and applying the same principles to a solid mass, as for example a piece of rock, it can be seen that restraint can be produced by causing a disturbance to change the dimensions of one portion of the rock relative to another portion by such an amount that the fracture strain in tension of the material in the restraining region is exceeded.

A local expansion in the material such as discussed above may result from any one or more or' the following phenomena: (l) an expansion or contraction due to change in temperature; (2) an expansion or contraction due to the piezo-electric effect; (3) an expansion or contraction resulting from magnetostriction. lt will be apparent that the field gradient of any of these phenomena must be steep enough to produce a strain exceeding the fracture strain of the material.

For example, assuming that the expansion desired is produced by an increase in temperature and that the ma-V terial to be fractured is quartz conglomerate, the tensile strength of this material is 242 pounds per square inch and the modulus of elasticity is 1763x106 pounds to the square inch. The thermal coefficient of expansion of this material is 10X106 inches per inch per degree centigrade.

From the foregoing we may determine that the fracture strain equals:

tensile strength I'" 5 modulus of elasticity 131.2/{10 inches per inch fracture strain thermal eoecient of expansion* 13.7 degrees C. or 24.6 degrees F From these calculations it will be observed that with the proper temperature gradient, tensile fracture can beproduced and that the difference required is of a very low order of magnitude, which is an important point when heating costs are considered.

We have found that Taconite ore can be fractured by subjecting the rock to high frequency energy. We have found that energy on the order of 25 kilowatts at a frequency of about 4 to 7 megacycles per second will successfully fracture Taconite.

By way of example, and using a 25 kilowatt high frequency induction unit of about 5 megacycles employing a high frequency current transformer on the output, we have broken up a block of Upper Cherty Taconite approximately 15 inches by 15 inches by l5 inches. Such a piece of rcel: is shown in Figure 1 at 10. The working coil in the example was constituted of a single turn with square legs of B; inch diameter water cooled copper tubing. The coil or loop is indicated in Fig. l at l1. The-legs of the loop 11 were about 16 inches on a side so as to give a coupling space of between 1/2 inch and 1 inch. The unit was adjusted for 2.7 to 2.9 plate amperes and 200 to 250 grid milliamperes. As soon as the high frequency energy was applied the rock began cracking immediately. Further time under load enlarged the cracks and spalling began to take place. The cracks appeared not only parallel to the strata but also perpendicular to it and the broken portions had a generally cubic form. Brightly heated regions appeared from the beginning of the application of power and increased in area and temperature with time. After considerable time under load under the coupling conditions used, an eruption of molten material took place. The piece of rock in question, after the application of the high frequency energy had the appearance of the piece a of Figure 2. The cracks in planes parallel to the strata are indicated at 12, and cracks 13 appear perpendicular thereto.

We found that this high frequency treatment resulted in a loss of strength of the Taconite and that the treated ore could be crumbled with light blows of a hammer. As distinguished from this, untreated Taconite rock can be broken only with the very greatest diiculty with hammer blows. We believe that this effect is due tothe formation of cracks, or chemical alterations in the bonding media.

In Figures 3 and 4 We have shown a continuous operation wherein rocks which may result from a fracture according to Figure 2 are further fractured. By way of example, We have shown a bench 14 upon which there is mounted a conveyor. An asbestos belt 15 passes over rollers 16 and 17 which are journaled in journals mounted upon the bench. The belt is driven by the rollers 1.8

clarity, but it will be understood that the belt being flexible supporting means must be provided for the upper flight of the belt.

Fragments of untreated ore are indicated at 24 as they are about to pass through a two turn coil unit indicated at 26, and it will be observed that in miniature it is similar to the fragment of Figure 2. In actual practice as the block 26 falls off the end of the conveyor it shatters into small pieces as indicated by the pile of fragments on the floor at 27.

it is not necessary that the fragments to be treated pass through a coil or loop and they may as Well pass in close proximity to a so-called pancake coil. These pancake coils Vare indicated at 2S and while We have shown one above the conveyor, one below the conveyor and one at each side of the conveyor, this is not neces- Y sary. With such an arrangement however it is possible to subject the rock to a series of surges of high frequency energy at different frequencies which may be desirable for some purposes. The coil 25 in Figure 3 consisted of two turns of S; inch copper tubing 5 inches in diameter. The belt 15 was driven at a speed from 5 to l0 feet a minute. The drop from the end of the conveyor to the floor was about 30 inches. In this example the coupling was such that a plate current of 2.9 amperes and a grid current of 200 milliamperes was obtained with the coil 25 about half full of the material. Successful coupling was obtained both by proximity as with coils such as the coils 28` and by encirclement by means of loop coils such as the coils 25.

In Figure 6 we have shown by way of example a mining operation. A stratum of iron bearing ore is shown at 3h. The overburden has been removed to expose a stratum of ore. At 31 we have shown a prime mover in the form of a Caterpillar tractor which is provided with a high frequency generating unit and which is provided with a pancake type of coil 32. The coil 32 is so arranged that as the prime mover 31 moves over the stratum the coil 32 passes in close proximity over the exposed surface of the ore. During this travel high frequency energy is generated and the ore is continuously or intermittently subjected totensile stresses beyond the fracture strain of the ore. This produces in an upper layer of the stratum a condition similar to that shown in Figure 2 so that thereafter a bulldozer, as indicated at 33, can simply scrape the fractured ore layer away to expose anotherY untreated layer. If the operation is at the side of a cliff or hill the bulldozer 33 may simply push the fractured layer over the edge of the hill so that in falling down it will be further fractured and comminuted as indicated by the pile 34. In some instances, it is advantageous to induce mechanical vibrations Vin the ore body. This would assist in the loosening and separation of the smaller pieces into which the body has been cracked by the electrical energy.

it will will understood that the fracture operation achieved according to our invention results from producing localized expansion so as to subjectadjacent areas of the rock to tensile stresses beyond the fracture strain thereof.

The expansion achieved may result from a temperature increase so that there is throughout the rock a differential heat effect. The dimensional changesrinvolved may result from magnetostrictive effects and eddy current effects, as Well as from piezo-electric effects. Probably more than one of these effects are involved, but we do not wish to be bound by any theory as to which of these phenomena is the most important.l

It will be clear that by the methods outlined above it will be possible to separate materials by selective fractur ing when such materials have different piezo-electric, magnetostrictive, thermal or electrical properties.

While we have detailed certain examples hereinabol e, it is to be understood that we are not limited to the application of energy of any one frequency since in some cases it may be desirable to apply varying frequencies or frequencies of two different values, either simultaneously or successively.

Similarly while we have shown a mining operation of the open type, it will be clear that the same principles can be applied to deep pit mining or tunnel approached mining or other enclosed types of mines. It will also be understood that the high frequency energy can be applied to a lcontinuous stream of the'mineral bearing bodies in blocks such as shown in Figure 3 as well as in smaller particle sizes or even in powder form or suspended form in a fluid for the purpose of disintegrating, reducing in size or beneficiating the material. lt will be understocd also that the dropping of the materials off the conveyor onto the oor is simply exemplary of any mechanical method or operation for further breaking down the size of the rock which has been weakened by the application of high frequency energy.

We therefore do not intend to limit ourselves in any manner other than as set forth in the claims which follow.

Having now fully described our invention, what we claim as new and desire to secure by Letters Patent is:

l. The method of mining Taconite ores wherein particles of magnetic iron oxide are distributed throughout the matrix, which includes the steps of removing the overburden from the ore to be mined, subjecting said magnetic iron oxide, in situ, to high frequency magnetic energency, to produce volumetric changes resulting in tensile strains beyond the fracture strain thereof, and scraping away the layer of ore so treated.

2. The method of mining Taconite ores wherein particles of magnetic iron oxide are distributed throughout the matrix, which includes the steps of removing the over-burden from the ore to be mined, and then alternately subjecting said magnetic iron oxide, in situ, to high frequency magnetic energy, to produce volumetric changes resulting in tensile strains beyond the fracture strain thereof, and removing the layer of ore so treated to expose additional untreated ore.

3. The method of mining Taconite ores wherein particles of magnetic iron oxide are distributed throughout the matrix, which includes the steps of removing the over-burden from the ore to be mined, subjecting the magnetic iron oxide, in situ, to high frequency magnetic energy to cause volumetric changes resulting in portions of said ore being subjected to tensile strains beyond the fracture strain thereof, and scraping away the layer of ore so treated.

4. The method of mining Taconite ores wherein particles of magnetic iron oxide are distributed throughout the matrix, which includes the steps of removing the over-burden from the ore to be mined, subjecting the inagnetic iron oxide, in situ, to high frequency magnetic energy to cause volumetric changes resulting in portions of said ore being subjected to tensile strains beyond the fracture strain thereof.

5. The method of mining Taconte ores wherein particles of magnetic iron oxide are distributed throughout the matrix, which includes the steps of removing the over burden from the ore to be mined, and then alternately subjecting the magnetic iron oxide, in situ, to high frequency magnetic energy to cause volumetric changes resuting in portions of said ore being subjected to tensile strains beyond the fracture strain thereof, and removing the layer of ore so treated to expose additional untreated ore.

6. The method of claim 3, wherein the high frequency magnetic energy is at a frequency of the order of 4 to 7 megacycles per second.

7. The method of claim 4, wherein the high frequency magnetic energy is at a frequency of the order of 5 megacycles per second.

8. The method of fracturing Taconite ores wherein particles of magnetic iron oxide are distributed throughout the matrix, which includes the steps of subjecting said magnetic iron oxide selectively to a field of high frequency magnetic energy to cause volumetric changes resulting in portions thereof being subjected to tensile strains beyond the fracture strain thereof, and thereafter mechanically breaking said rock into small pieces.

9. The method of claim 8, wherein said ores are conveyed through an inductor of electrically conductive material and a high frequency electric current is applied to said inductor.

10. The method of claim 8, wherein said ores are conveyed through a loop of electrically conductive material and an electric current at a frequency of about five megacycles per second is applied to said loop.

11. The method of claim 8, wherein said ores are conveyed in close proximity to a loop of electrically conductive material and wherein a high frequency electric current is applied to said loop.

12. The method of claim 8, wherein said ores are conveyed in close proximity to an inductor of electrically conductive material and wherein an electric current at a frequency of about four to seven megacycles per second is applied to said inductor.

References Cited in the file of this patent UNITED STATES PATENTS 44,986 Spencer Nov. 8, 1864 309,766 Christiansen Dec. 23, 1884 502,181 Fauvel July 25, 1893 561,367 Badger June 2, 1896 1,383,963 Mordey July 5, 1921 1,401,445 Rodgers Dec. 27, 1921 1,631,290 Sheedy et al `lune 7, 1927 1,640,524 Augustine Aug. 30, 1927 1,679,857 France Aug. 7, 1928 1,830,934 Curtis Nov. 10, 1931 2,106,869 Falkenstein Feb. 1, 1938 2,286,191 Aitchison June 16, 1942 2,308,860 Clark Ian. 19, 1943 2,444,259 Jordan June 29, 1948 2,517,425 Grumel Aug. 1, 1950 2,571,643 Youhouse Oct. 16, 1951 2,601,421 Thaning June 24, 1952 2,628,785 Fink Feb. 17, 1953 FOREIGN PATENTS 918,321 France Oct. 21, 1946 

8. THE METHOD OF FRACTURING TACONITE ORES WHEREIN PARTICLES OF MAGNETIC ION OXIDE ARE DISTRIBUTED THROUGHOUT THE MATRIX, WHICH INCLUDES THE STEPS OF SUBJECTING SAID MAGNATIC IRON OXIDE SELECTIVELY TO A FIELD OF HIGH FREQUENCY MAGNETIC ENERGY TO CAUSE VOLUMETRIC CHANGES RESULTING IN PORTIONS THEREOF BEING SUBJECTED TO TENSILE STRAINS BEYOND THE FRACTURE STRAIN THEREO, AND THATEAFTER MECHANICALLY BREAKING SAID ROCK INTO SMALL PIECES. 